1. Field of the Invention
This invention relates generally to portable radio sets and, more particularly to those of the type having a built-in microphone-speaker and capable of being carried on a person as at his waist with the aid of a carrying holder with a separate microphone-speaker connected to the radio unit exteriorly thereof.
2. Description of the Prior Art
Generally, a portable radio set of the type concerned can serve with its internal or built-in microphone-speaker as a radio transmitter-receiver unit without involving any trouble, even if no external microphone-speaker is employed thereon. It can also serve the function of radio transmission and reception with a small-sized, external microphone-speaker connected by cable means to the radio unit besides the internal microphone-speaker. Use of such an external microphone-speaker is particularly desirable in cases where the radio set is used for an extended period of time or when the user's hands are full in some operation, since the radio set can be carried on the person as at his waist with the aid of a carrying device and the external microphone-speaker be secured, for example, to the lapel of his coat.
On the other hand, it is critically important with portable radio sets of the type concerned to minimize the power consumption without impairing the performance of the radio set. Heretofore, however, in cases where the radio is used with an external microphone-speaker connected thereto, the latter is connected in parallel or in series with the internal microphone-speaker. As the result, the total power consumption is increased in the case of parallel connection and the output sound level of each microphone-speaker is reduced in the case of series connection. Further, when one of the microphone-speakers is used as a microphone, noise from the other unused ones adversely affects the sound to be transmitted because both microphone-speakers are electrically connected to the transmitter.